Use .str() instead of .data() and .size() when converting CDataStream to
a string. Uses std::string, avoiding conversion to a C string.

Github-Pull: bitcoin#14588
Rebased-From: fe5d22b

Github-Pull: bitcoin#14588
Rebased-From: 4f3f5cb

New constructor that creates a PartiallySignedTransaction from a
CTransaction, automatically sizing the inputs and outputs vectors for
convenience.

Github-Pull: bitcoin#14588
Rebased-From: 65166d4

Refactor out a "PSBTInputSigned" function to check if a PSBT is signed,
for use in subsequent commits.

Also improve a related comment.

GitHub-Pull: bitcoin#14588
Rebased-From: 53e6fff

Remove redundant arguments to SignPSBTInput -- since it needs several
bits of the PartiallySignedTransaction, pass in a reference instead of
doing it piecemeal. This saves us having to pass in both a PSBTInput and
its index, as well as having to pass in the CTransaction. Also avoid
redundantly passing the sighash_type, which is contained in the
PSBTInput already.

Github-Pull: bitcoin#14588
Rebased-From: 0f5bda2

Refactor the process of PSBTInput signing to enforce the invariant that
a PSBTInput always has _either_ a witness_utxo or a non_witness_utxo,
never both.

This simplifies the logic of SignPSBTInput slightly, since it no longer
has to deal with the "both" case. When calling it, we now give it, in
order of preference: (1) whichever of the utxo fields was already
present in the PSBT we received, or (2) if neither, the
non_witness_utxo field, which is just a copy of the input transaction,
which we get from the wallet.

SignPSBTInput no longer has to remove one of the two fields; instead, it
will check if we have a witness signature, and if so, it will replace
the non_witness_utxo with the witness_utxo (which is smaller, as it is
just a copy of the output being spent.)

Add PSBTInput::IsSane checks in two more places, which checks for
both utxo fields being present; we will now give an RPC error early on
if we are supplied such a malformed PSBT to fill in.

Also add a check to FillPSBT, to avoid touching any input that is
already signed. (This is now redundant, since we should no longer
potentially harm an already-signed input, but it's harmless.)

fixes bitcoin#14473

Github-Pull: bitcoin#14588

Github-Pull: bitcoin#14588
Rebased-From: e13fea9

Github-Pull: bitcoin#14197
Rebased-From: 862d159